The present invention relates to the structure of a thin-film electroluminescent (referred to as "EL" hereinafter) display panel and, more particularly, to a layer structure of a thin-film electroluminescent display panel.
Firstly, a conventional electroluminescent (EL) display panel is illustrated in FIG. 1, wherein the EL display panel comprises a first transparent glass substrate 1, a transparent electrode 2 made of In.sub.2 O.sub.3, SnO.sub.2 etc. formed thereon, a first dielectric layer 3 made of Y.sub.2 O.sub.3, TiO.sub.2, Si.sub.3 N.sub.4, SiO.sub.2, etc., an EL thin film 4 made of ZnS:Mn, and a second dielectric layer 5 made of a similar material of the first dielectric layer 3. A counter electrode 6 is made of Al and is formed on the second dielectric layer 5 through evaporation techniques. The first dielectric layer 3 is provided by sputtering or electron beam evaporation techniques. The EL thin film 4 is made of a ZnS thin film doped with manganese at a desired amount.
An AC electric field from an AC power source 7 is applied to the transparent electrode 2 and the counter electrode 6 to activate the EL thin film 4.
The EL thin film 4 is fabricated by electron beam evaporating a ZnS sintered pellet doped with Mn at a preferable quantity and, then, by heat-treating it in a vacuum or an inert gas atmosphere. Mn serves as a luminescent center in the EL thin film 4. The above EL display panel is characterized in that a conductive current does not flow into the EL thin film 4, but a displacement current flows within the film 4 in the form of the drift of free electrons therein, in order to emit electroluminescence from the film 4. The dielectric properties of the first and the second dielectric layers 3 and 5 surrounding the film 4 are very critical to the reliability of the EL display panel. The dielectric properties are to be able to withstand a high applied voltage.
The dielectric properties of the EL display panel depend on the thickness of the first and the second dielectric layers 3 and 5, in addition to the substances of these layers. To increase the dielectric properties of the layers, it is preferable that the layers are made thick.
However, as the dielectric layers 3 and 5 are made thick, a high operating voltage is needed. FIG. 2 shows a graph representing the relation of electroluminescence brightness VS. applied voltages of the EL display panel, where an l.sub.1 curve is related to the thickness of the layers thinner than that of the layers related to an l.sub.2 curve. As shown in this graph, the rising of the electroluminescence brightness in response to the increase of the applied voltage is made slower as the thickness of the layers are larger.
This indicates that the total thickness of the first and the second dielectric layers 3 and 5 should be limited to some extent within which the dielectric properties of these layers must be improved.
In addition, the dielectric properties of the dielectric layers 3 and 5 depend greatly on the surface condition, namely, the smoothness of the substrate for supporting the layers. The first dielectric layer 3 is supported on the surface of the transparent electrode 2. The substrate for the second dielectric layer 5 is the surface of the EL thin film 4. Comparing the surfaces of the electrode 2 and the film 4, the surface of the electrode 2 is smoother than that of the film 4.
More particularly, since the electrode 2 is formed on the very smooth surface of the transparent glass substrate 1 in a small thickness, the surface of the electrode 2 is very smooth. On the other hand, the film 4 is formed on a plurality of thin layers so that the surface condition of the film 4 depends on the surface conditions of all these thin layers, in totality. Further, the film 4 comprises a polycrystalline film having large grain sizes in a large thickness, so that the surface of the film 4 is not smooth, but rather rugged due to the condition that a plurality of pin holes which are very uneven are present.
Therefore, in a case where the dielectric layers 3 and 5 are formed with the same manufacture conditions, the dielectric properties of the second dielectric layer 5 are worse than those of the first dielectric layer 3.
In conclusion, the conventional layers are not suitable for the thin-film EL display panel to assure appropriate dielectric properties of the layers.